This invention relates to a phase lock loop circuit, particularly to a phase lock loop circuit which locks a phase of a recovered carrier to the received carrier of PSK (Phase-Shift-Keying) signal.
In a synchronous detection system for demodulating a PSK signal, a carrier being phase-locked to a carrier of a received PSK signal must be recovered. Therefore, in an existing synchronous detecting system of this type, an output obtained by detecting the received PSK signal with a recovered carrier is subjected to logical processing in order to remove the phase component. Then, the output signal is applied to a voltage controlled oscillator as a phase error signal and the phase of the recovered carrier output from the voltage controlled oscillator is locked to the phase of the received PSK signal.
In an ordinary phase lock loop circuit, a high performance loop which is less influenced by disturbance can be obtained by making the loop bandwidth narrower, but an excessively narrow loop bandwidth will invite deterioration of the tracking characteristics for the received PSK signal. Accordingly, a method for ensuring stable circuit operation with an excellent tracking characteristic and a narrow loop bandwidth has been proposed, wherein the specified sweep signal is superimposed on the phase error signal in order to control the voltage controlled oscillator until the synchronous operation or circuit is established and the supply of the sweep signal is suspended after the synchronous operation is established.
However, the known method for superimposing such sweep signal results in an extra sweep signal being generated at the time the superimposition of such sweep signal is suspended, thereby causing a disturbance to the phase lock loop. It has, therefore been desired to provide a phase lock loop which does not generate an unwanted sweep signal component and requires only a simple circuit structure.